Our world orbits one star. There are many multiple star systems and some binary stars have been found with planets. Now how about a star system with four suns and some planets? For the first time a planet has been found that orbits one pair of stars and has a second pair of stars revolving around it — so that four stars illuminate its skies. The discovery was made by volunteers using the planethunters.org website alongside a team led by astronomers from Oxford University, Yale University, and Adler Planetarium. Whilst binary stars — systems with pairs of stars — are not uncommon, out of the thousands of planets discovered outside our solar system until now only six* had been found orbiting binary stars (circumbinary planets) and none of these are known to have another pair of stars circling them until now.

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The planet, dubbed PH1 after the Planethunters.org website, is thought to be a gas giant slightly larger than Neptune but over six times the size (radius) of the Earth, located just under 5,000 light-years away in the constellation of Cygnus. Quite how such a four star system remains stable enough not to tear PH1 apart is a mystery.

PH1 orbits outside the 20-day orbit of a pair of eclipsing stars that are 1.5 and 0.41 times the mass of the Sun. It revolves around its host stars roughly every 138 days. Beyond the planet’s orbit at about 1000 AU (roughly 1000 times the distance between Earth and the Sun) is a second pair of stars orbiting the planetary system.

The volunteers — Kian Jek of San Francisco and Robert Gagliano of Cottonwood, Arizona — spotted faint dips in light caused by the planet as it passed in front of its parent stars, a common method of finding extrasolar planets. Schwamb, a Yale postdoctoral researcher, led the team of professional astronomers that confirmed the discovery. PH1 is a gas giant with a radius about 6.2 times that of Earth, making it a bit bigger than Neptune.

"It's fascinating to try and imagine what it would be like to visit a planet with four Suns in its sky, but this new world is confusing astronomers - it's not at all clear how it formed in such a busy environment," said Dr Chris Lintott, of Oxford University, one of the Planethunters.org team.

Meg Schwamb of Yale University, lead author of an upcoming paper on the discovery, is presenting the results on 15 October at the Division for Planetary Sciences annual meeting in Reno, Nevada. She said: "The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments."

Kian Jek said: "It still continues to astonish me how we can detect, let alone glean so much information, about another planet thousands of light years away just by studying the light from its parent star."